1. Field of the Invention
The present invention relates to a variable-focal-length lens system for projection and a projection apparatus. More particularly, the invention relates, for example, to a variable-focal-length projection lens system with a magnification varying function suitable for enlarged projection of an image displayed on an image display device, such as a digital micromirror device or an LCD (liquid crystal display), onto a screen, and to a projection apparatus provided with such a variable-focal-length projection lens system.
2. Description of Related Art
Many projection apparatus for business use, in particular digital cinematographic projection apparatus, adopt a focusing method involving forward shifting of an entire projection lens system. This projection method involving the forward shifting of an entire lens system has the disadvantage of requiring a large focusing mechanism for moving the large, heavy lens system as a whole, though, on the other hand, it also has the advantage of achieving satisfactory focus even when the back focal length of the projection lens system is slightly deviated from the design value. Thus, the entire-system forward shifting projection method is considered suitable for business use.
Inconveniently, however, the entire-system forward shifting projection method mentioned above suffers from a number of problems in terms of optical performance. Specifically, the projection lens system is expected to provide satisfactory projection performance over the range of distance in which the projection distance may vary, but in reality, so long as the projection lens system is left intact, its performance deteriorates notably as the projection distance varies. Though depending on the size of the movie theater, when the projection distance varies from 45 m (remote projection) to 15 m (close projection), leaving the projection lens system intact brings a variation in curvature of field that amounts to 20 μm to 30 μm (as measured on the reduction-side image surface) to the over side (the over side here denotes the direction going away from the projection lens system).
Before, the size of each pixel on an image display device was so large that a variation in curvature of field as mentioned above did not pose a serious problem. Today, however, a 4K-compatible (4096×2160-pixel) image display device of the same chip size has a far larger number of pixels, and thus each pixel has one-half or less of the conventional size. Accordingly, a variation in curvature of field resulting from a variation in projection distance now poses a serious problem. Addressing the problem, Patent Document 1 listed below proposes a varifocal lens system devised for improved projection performance.    Patent Document 1: JP-A-2002-122782
The variable-focal-length projection lens system disclosed in Patent Document 1 is a varifocal projection lens system composed of four, namely a positive, a negative, a positive, and a positive, lens groups wherein, during magnification varying, the first lens group remains stationary while the second, third, and fourth lens groups move. During focusing, the entire projection lens system moves, and during magnification varying, the image surface moves greatly even with the projection distance constant. For example, in Example 1, the image surface (reduction-side) moves 2.2 mm at the maximum, and in Example 2, the image surface (reduction-side) moves 17 mm at the maximum. Here, increasing flexibility in design results in enhanced projection performance, but no measures are taken against a variation in curvature of field resulting from a variation in projection distance, with the result that, as the projection distance varies from 45 m (remote projection) to 15 m (close projection), curvature of field varies about 20 μm to the over side. Such notable deterioration in projection performance resulting from a variation in projection distance makes the projection performance unsatisfactory in projection onto screens of varying sizes, from large to small, using recent high-definition image display devices.